SUBSIDIARY GUARANTY SUPPLEMENT

EXHIBIT 10.42

EXECUTION VERSION

SUBSIDIARY GUARANTY SUPPLEMENT

February 5, 2008

Bank of America, N.A., as Administrative Agent

MA5-100-11-02

100 Federal Street

Boston, MA 02110

Telecopy: (617)  ###-###-####

Attention: Bill Faidell, Agency Management Office

Revolving Credit Agreement dated as of March 31, 2005 (as amended, amended and restated,

supplemented or otherwise modified from time to time) among

Mustang MSC-Florida Acquisition, Inc., a Florida corporation (the “Initial Borrower”),

MSC-Medical Services Company, a Florida corporation (as the

surviving entity of the Merger, the “Borrower”), the Lenders

party to the Revolving Credit Agreement, and Bank of America, N.A.,

as Administrative Agent and L/C Issuer

Ladies and Gentlemen:

Reference is made to the above-captioned Revolving Credit Agreement and to the Subsidiary Guaranty referred to therein (such Subsidiary Guaranty, as in effect on the date hereof and as it may hereafter be amended, supplemented or otherwise modified from time to time, together with this Guaranty Supplement, being the “Subsidiary Guaranty”). The capitalized terms defined in the Subsidiary Guaranty or in the Revolving Credit Agreement and not otherwise defined herein are used herein as therein defined.

Section 1. Guaranty; Limitation of Liability. (a) The undersigned hereby absolutely, unconditionally and irrevocably guarantees the punctual payment when due, whether at scheduled maturity or on any date of a required prepayment or by acceleration, demand or otherwise, of all Obligations of each other Loan Party now or hereafter existing under or in respect of the Loan Documents (including, without limitation, any extensions, modifications, substitutions, amendments or renewals of any or all of the foregoing Obligations), whether direct or indirect, absolute or contingent, and whether for principal, interest, premium, fees, indemnities, contract causes of action, costs, expenses or otherwise (such Obligations being the “Guaranteed Obligations”), and agrees to pay any and all expenses (including, without limitation, fees and expenses of counsel) incurred by the Administrative Agent or any other Secured Party in enforcing any rights under this Guaranty Supplement, the Subsidiary Guaranty or any other Loan Document. Without limiting the generality of the foregoing, the undersigned’s liability shall extend to all amounts that constitute part of the Guaranteed Obligations and would be owed by any other Loan Party to any Secured Party under or in respect of the Loan Documents but for the fact that they are unenforceable or not allowable due to the existence of a bankruptcy, reorganization or similar proceeding involving such other Loan Party.

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(b) The undersigned, and by its acceptance of this Guaranty Supplement, the Administrative Agent and each other Secured Party, hereby confirms that it is the intention of all such Persons that this Guaranty Supplement, the Subsidiary Guaranty and the Obligations of the undersigned hereunder and thereunder not constitute a fraudulent transfer or conveyance for purposes of Debtor Relief Law, the Uniform Fraudulent Conveyance Act, the Uniform Fraudulent Transfer Act or any similar foreign, federal or state law to the extent applicable to this Guaranty Supplement, the Subsidiary Guaranty and the Obligations of the undersigned hereunder and thereunder. To effectuate the foregoing intention, the Administrative Agent, the other Secured Parties and the undersigned hereby irrevocably agree that the Obligations of the undersigned under this Guaranty Supplement and the Subsidiary Guaranty at any time shall be limited to the maximum amount as will result in the Obligations of the undersigned under this Guaranty Supplement and the Subsidiary Guaranty not constituting a fraudulent transfer or conveyance.

(c) The undersigned hereby unconditionally and irrevocably agrees that in the event any payment shall be required to be made to any Secured Party under this Guaranty Supplement, the Subsidiary Guaranty, the Guaranty by Holdings set forth in the Revolving Credit Agreement or any other guaranty or guaranty supplement delivered pursuant to Section 6.12 of the Revolving Credit Agreement, the undersigned will contribute, to the maximum extent permitted by applicable law, such amounts to each other Guarantor and each other guarantor so as to maximize the aggregate amount paid to the Secured Parties under or in respect of the Loan Documents.

Section 2. Obligations Under the Guaranty. The undersigned hereby agrees, as of the date first above written, to be bound as a Guarantor by all of the terms and conditions of the Subsidiary Guaranty to the same extent as each of the other Guarantors thereunder. The undersigned further agrees, as of the date first above written, that each reference in the Subsidiary Guaranty to an “Additional Guarantor” or a “Guarantor” shall also mean and be a reference to the undersigned, and each reference in any other Loan Document to a “Subsidiary Guarantor” or a “Loan Party” shall also mean and be a reference to the undersigned.

Section 3. Representations and Warranties. The undersigned hereby makes each representation and warranty set forth in Section 6 of the Subsidiary Guaranty to the same extent as each other Guarantor.

Section 4. Delivery by Telecopier. Delivery of an executed counterpart of a signature page to this Guaranty Supplement by telecopier shall be effective as delivery of an original executed counterpart of this Guaranty Supplement.

Section 5. Governing Law; Jurisdiction; Waiver of Jury Trial, Etc. (a) This Guaranty Supplement shall be governed by, and construed in accordance with, the laws of the State of New York.

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(b) The undersigned hereby irrevocably and unconditionally submits, for itself and its property, to the nonexclusive jurisdiction of any New York State court or any federal court of the United States of America sitting in New York City, and any appellate court from any thereof, in any action or proceeding arising out of or relating to this Guaranty Supplement, the Subsidiary Guaranty or any of the other Loan Documents to which it is or is to be a party, or for recognition or enforcement of any judgment, and the undersigned hereby irrevocably and unconditionally agrees that all claims in respect of any such action or proceeding may be heard and determined in any such New York State court or, to the extent permitted by law, in such federal court. The undersigned agrees that a final judgment in any such action or proceeding shall be conclusive and may be enforced in other jurisdictions by suit on the judgment or in any other manner provided by law. Nothing in this Guaranty Supplement or the Subsidiary Guaranty or any other Loan Document shall affect any right that any party may otherwise have to bring any action or proceeding relating to this Guaranty Supplement, the Subsidiary Guaranty or any of the other Loan Documents to which it is or is to be a party in the courts of any other jurisdiction.

(c) The undersigned irrevocably and unconditionally waives, to the fullest extent it may legally and effectively do so, any objection that it may now or hereafter have to the laying of venue of any suit, action or proceeding arising out of or relating to this Guaranty Supplement, the Subsidiary Guaranty or any of the other Loan Documents to which it is or is to be a party in any New York State or federal court. The undersigned hereby irrevocably waives, to the fullest extent permitted by law, the defense of an inconvenient forum to the maintenance of such suit, action or proceeding in any such court.

(d) THE UNDERSIGNED HEREBY IRREVOCABLY WAIVES, TO THE FULLEST EXTENT PERMITTED BY APPLICABLE LAW, ALL RIGHT TO TRIAL BY JURY IN ANY ACTION, PROCEEDING OR COUNTERCLAIM (WHETHER BASED ON CONTRACT, TORT OR OTHERWISE) ARISING OUT OF OR RELATING TO ANY OF THE LOAN DOCUMENTS, THE REVOLVING CREDIT LOANS, LETTERS OF CREDIT OR THE ACTIONS OF ANY SECURED PARTY IN THE NEGOTIATION, ADMINISTRATION, ADVANCING, ISSUANCE, PERFORMANCE OR ENFORCEMENT THEREOF.

[signature on following page]

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Signature Page to Guaranty Supplement

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