Adolescent with Duchenne muscular dystrophy

Correct answer: A. 0%

• A. CORRECT. Duchenne muscular dystrophy is X-LINKED RECESSIVE. The daughter receives one X from her father and one from her mother. Her father is UNAFFECTED, so the X he transmits carries a normal allele, and that normal allele is dominant over the mutant one. She therefore CANNOT be phenotypically affected, whatever she inherits from her mother. Her risk of being a CARRIER is 50 percent, which is the number the question is baiting you toward.
• B. 100 percent would only apply if both her X chromosomes necessarily carried the mutation — impossible with an unaffected father.
• C. 25 percent would be the affected-offspring figure for an AUTOSOMAL RECESSIVE condition with two carrier parents. That is a different inheritance pattern.
• D. 50 percent is the probability that she is a CARRIER, not that she is affected. Confusing carrier status with affected status is the single commonest error on X-linked pedigree questions.

Note: The full cross — carrier mother XᴿXʳ by unaffected father XᴿY: sons are 50% affected, daughters are 50% carriers and 0% affected. A small minority of female carriers show mild symptoms through skewed X-inactivation, but that is not what 'phenotypically affected' means on an exam.

Correct answer: B. Inferior alveolar nerve block

• A. An intrapapillary injection anaesthetises the interdental papilla for rubber dam or matrix placement. It provides no pulpal anaesthesia.
• B. CORRECT. Mandibular molars require an INFERIOR ALVEOLAR NERVE BLOCK. The dense cortical bone of the adult mandible in the molar region does not allow reliable diffusion from a buccal infiltration, so the nerve must be anaesthetised proximally, at the mandibular foramen, before it enters the canal. A long buccal supplement is added for the buccal soft tissue during extraction.
• C. The posterior superior alveolar block covers the MAXILLARY molars. Wrong arch.
• D. Buccal infiltration alone is reliable in the MAXILLA, where the cortical plate is thin and porous, and in the mandible only for primary teeth and permanent incisors. It fails at mandibular molars.

Note: Duchenne-specific considerations: progressive respiratory muscle weakness and impaired cough make general anaesthesia and deep sedation genuinely hazardous, so local anaesthesia in the chair is preferred where possible. Cardiomyopathy is near-universal by adolescence, so a cardiology review is worthwhile before extensive care, and the patient may need support to maintain an upright, safe airway position.